The present invention relates to a fuel supply control system for an internal combustion engine.
There is well known a fuel supply control system employing a feedback control scheme wherein the air-fuel ratio of the combustible air-fuel mixture supplied to the engine is monitored by an air-fuel ratio detecting sensor such as, for example, an O.sub.2 sensor so that the air-fuel ratio can be controlled to a predetermined value, that is, a stoichiometric value. According to the disclosure of the Japanese Laid-open Patent Publication No. 58-72631, published Apr. 30, 1983, the feedback control of fuel supply is so designed that, upon and after the establishment of a condition requiring the feedback control, the air-fuel ratio of the combustible mixture supplied at low load, low speed operating condition can be controlled in dependence on the load on the engine so as to provide a leaned combustible mixture, but the combustion gases to be supplied during an engine operating condition other than the low, load, low speed engine operating condition can be controlled to the stoichiometric air-fuel ratio.
During the acceleration of the engine, in order to ensure a high power output required during such time, the feedback control described above is interrupted to permit the increased fuel supply to render the air-fuel ratio to be of a low value, for example, 13.
However, the rate of increase of fuel in this case has hitherto been fixed at a particular value, and considering the acceleration from the engine operating condition wherein the feedback control is being effected to provide the air-fuel mixture of stoichiometric ratio, the increased fuel supply according to the fixed rate of increase of fuel is effective to achieve a required characteristic of acceleration because the difference is small between the air-fuel ratio (stoichiometric air-fuel ratio of 14.7) shortly before the start of acceleration and the desired air-fuel ratio (for example, 13) during the acceleration. This two-stage air-fuel control system however has a problem. More particularly, where acceleration is desired to be started from the low load, low speed operating condition of the engine, a considerable increase of the charge incident to the start of acceleration may result in the supply of fuel in a quantity short of the required quantity because the air-fuel ratio shortly before the start of acceleration is high, the consequence of which is the temporal supply of excessively leaned air-fuel mixture. Once this happens, hesitation of acceleration occur in the engine and, in the worst case it may happen, the misfiring ofthe air-fuel mixture may occur.